From the earlier application mentioned above, and the references cited therein, it is known to produce silicon or ferrosilicon by a method in which raw-material blanks are first formed, which contain fine-grain silicon-dioxide and carbon in excess in respect of the reduction to silicon carbide and the raw-material blanks are introduced into a low-shaft electric furnace as a charge in mixture with silicon dioxide in lump form.
The silicon dioxide in the raw-material moldings (usually briquettes) is reduced to silicon carbide in an upper part of the electric low-shaft furnace at a temperature of below 1600.degree. C. and coke structure agglomerates are formed from the raw-material molding carbon unused during this reduction.
The molten silicon dioxide added in lump form with the charge is reduced, with the silicon carbide and carbon from the coke structure agglomerates, to silicon in a bottom part of the electric low shaft furnace at a temperature of above 1600.degree., preferably from 1800.degree. to 2000.degree. C.
"Silicon dioxide" as this term is used herein denotes all conventional silicon carriers, more particularly, quartzite and quartz sand.
"Fine grain" means as fine as sand, particle size, for example, 0.5 to 5 mm, preferably about 1 mm.
"Excess" means that the carbon unused in the raw-material moldings for the reduction of the silicon dioxide to silicon carbide is quantitatively sufficient to stoichiometrically produce the SiC and leave enough carbon to form the coke structure agglomerates.
Considered summarily, the reduction takes place in two stages as follows: EQU SiO.sub.2 +3C=SiC+2CO, EQU SiO.sub.2 +2SiC=3Si+2CO.
In the second stage, it progresses to form silicon monoxide: EQU SiO.sub.2 +C=SiO+CO.
The silicon monoxide, which is gaseous at the temperature concerned, reaches the top of the low-shaft electric furnace.
In earlier methods of the kind concerned (see the above-mentioned application and German Patent document DE-OS No. 34 11 371) the raw-material moldings are made by briquetting.
Briquettable carbon is used in a quantity sufficient for briquetting, preferably by hot briquetting, although cold briquetting is also used, with the addition of bituminous binders.
The raw-material moldings also contain the carbon in the form of carbon carriers, which are inert in respect of briquetting, e.g. petroleum coke, anthracite, graphite, lignite coke, coal coke and the like.
Silicon dioxide can, of course, be made into ferrosilicon and silicon metal by introducing suitable substances into the low-shaft electric furnace, e.g. iron in the form of iron shavings or iron granulate or iron oxide. These known steps have proved satisfactory. They give a considerably increased silicon yield with a low power consumption to the low-shaft electric furnace and a reduced electrode consumption for the same.
This is due, we have found, to the fact that coke structure agglomerates are formed from the raw-material moldings in the first reduction stage to silicon carbide, and they have a much larger surface, i.e. coke structure surface, as compared with the carbon in a silicon dioxide and carbon charge.
Their specific internal area is usually less than 5 m.sup.2 /g. The enlarged surface makes the coke structure agglomerates particularly reactive and to some extent activates them in respect of the carbon reactivity. However, still some gaseous silicon monoxide is liberated, and this has an adverse effect on the silicon yield and also the current consumption.